1. Field of the Invention
The present invention relates to a comb-shaped actuator used for such as an optical control element.
2. Description of Related Art
A mirror or a filter reflecting or permeating partially a light is used for optical resonators, photo-switches and optical attenuators in the optical fiber communication. Though the diameter of a light beam impinging into or outgoing from an optical fiber is as small as 1 mm or less, a mirror per se or a mechanism attached thereto for turning or shifting the mirror makes a device large compared to the light beam. Therefore, miniaturizing the above components is required.
As a structure for miniaturization, a comb-shaped actuator has been developed. That is, the actuator comprises a movable electrode having comb-shaped electrode fingers movable by an electrostatic attracting force at the periphery of a micro mirror and a fixed electrode having comb-shaped electrode fingers as well, both the electrodes being interdigitated together, and drives the movable electrode with an electrostatic force generated by a voltage applied between both the electrodes.
FIG. 13 explains a comb-shaped actuator in which a rectangular movable electrode 101 having movable electrode fingers 102a, 102b in the shape of teeth of a comb are arranged on two opposite sides thereof and a pair of fixed electrodes 110, 111 are provided facing the movable electrodes. Fixed electrode fingers 113a, 113b in the shape of teeth of a comb are provided on the sides of the fixed electrode facing the movable electrode, and the electrode fingers of both electrodes are combined together in an interdigital arrangement. Fixed portions 121, 122, which are located apart from the movable electrode and to act as connecting terminals of the movable electrode, are placed facing the other opposite sides 104, 105 of the movable electrode 101. Suspensions 106, 107 suspending the movable electrode 101 and connecting it with the fixed portions 121, 122 are provided at each center of the opposite sides 104, 105.
As shown in FIG. 14, the movable electrode fingers 102 of the movable electrode 101 and the fixed electrode fingers 113 of the fixed electrodes 110 (111) are located together in an interdigital arrangement. The movable electrode finger 102 intervening between a pair of neighboring fixed electrode fingers 1131, 1132 is located apart therefrom with gap regions 1311, 1312 of equal distance g0 respectively. When each electrode finger has the length l, the height h (h1, h2), and the width w shown in FIG. 15 and FIG. 16, each electrode finger receives an upward attracting force F generated by an electrostatic force E upon a voltage applied between both the electrodes if the movable electrode finger 102 is located below the upper fixed electrode fingers 1131, 1132 as shown in FIG. 15(A). When both the electrode fingers 102, 1131, 1132 are arranged on the same surface as shown in FIG. 15(B), an attracting force F in the direction of the height is generated subject to each electrode finger having different heights, e.g. the height of the fixed electrode finger being h1, and the height of the movable electrode finger being h2. As shown in FIG. 13, the movable electrode 101 turns around the suspension supporting members 106, 107 as shown by the mark 108.
However, if the electrostatic force generated in the gap region between the fixed electrode fingers 1131, 1132 and the movable electrode finger 102 shown in FIG. 14 does not act equally for the left and the right, a strong attracting force against the movable electrode finger acts on one of the fixed electrode fingers e.g. the electrode finger 1131. Even if manufacturing error of the electrode is minute, the electrostatic force is prone to be uneven. An uneven electrostatic force thus generated causes a rotating electrostatic force or an uneven electrostatic force toward one side in the movable electrode surface such as the mark 109a or 109b of FIG. 13 over whole electrode finger group. As a result, the fixed electrode and the movable electrode come into contact with each other and never separate from each other, thereby preventing predetermined turning control.